Apparatus for detecting incipient fire and explosion



Feb. 8, 1955 A. MATHISEN 2,701,827

APPARATUS FOR DETECTING INCIPIENT FIRE AND EXPLOSION Flled April 14, 1952 4 Sheets-Sheet l j FD --ED I u 24: 2 8 22 23 3; 27 em A. MATHISEN 2,701,827

APPARATUS FOR DETECTING INCIPIENT FIRE AND EXPLOSION F ch. 8, 1955 4 Sheets-Sheet 2 Filed April 14, 1952 Jr 49 as Feb. 8, 1955 A. MATHISEN 2,701,827

APPARATUS FOR DETECTING INCIPIENT FIRE AND EXPLOSION Filed April 14, 1952 4 Sheets-Sheet s A. MATHISEN Feb. 8, 1955 4 Sheets-Sheet 4 Filed April 14, 1952 12 MEN-re K M m .5 m z A 7 fi 3 fl 0mm m N x p M 0M 4 6 E w M w W r W d C a ./A

APPARATUS FOR DETECTING INCIPIENT FIRE AND EXPLOSION Anders Mathisen, London, England, assignor of one-half to The Wilkinson Sword Company Limited, London, England, a British company, and one-half to Graviner Manufacturing Company Limited, London, England, a British company Application April 14, 1952, Serial No. 282,243

Claims priority, application Great Britain April 30, 1951 21 Claims. (Cl. 200-33) This invention relates to apparatus for detecting rise in pressure due to an incipient fire or explosion.

British specification No. 643,188 describes fire or explosion detecting and suppressing apparatus comprising pressure rate of rise responsive diaphragm means for detecting rise in pressure due to an incipient fire or explosion, said diaphragm means being connected in an electric circuit including a contact adapted to be closed by said means to release a fire or explosion suppressing substance, the diaphragm means being selected to be of a high order of sensitivity such as to respond to a small pressure rise when the rate of pressure rise is in excess of 50 lbs. per sq. inch per second, and being capable of closing the contact within 0.02 second of the initiation of the pressure rise, whereby incipient fire or explosion can be detected before it is established.

In the application of appliances intended to detect and suppress explosions in aircraft fuel tanks difiiculties arise due to the fact that under some conditions the detecting appliance or appliances may be mounted in the vapour space, and under other conditions, it or they may be submerged below the liquid, and this seriously aifects the sensitivity of response to the relatively slow rise of pressure in the initial stages of an incipient fire or explosion.

An object of the present invention is to overcome the above disadvantage and to provide a detecting apparatus which may be mounted either in the vapour space or be submerged in the liquid fuel without affecting the sensitivity of the apparatus.

With the foregoing object in view, detecting apparatus made in accordance with the present invention comprises a pressure rate of rise responsive element and is characterised by the feature that the pressure rate of rise responsive element is mounted in a housing so as to provide chambers in said housing on opposite sides of said element, one of said chambers having two spaced movable walls, one of which is provided by the pressure rate of rise responsive element and the other of which is capable of transmitting pressure variations in the surrounding medium through said chamber to actuate the pressure rate of rise responsive element.

The detecting device may be used in combination with electrically operated explosion suppressing means, the arrangement being such that movement of the pressure rate of rise responsive element when the latter is operated actuates an electric circuit to control the electrical operation of the suppressing means.

The movable wall constituted by the pressure rate of rise responsive element may comprise a thin flexible diaphragm extending across the housing, and means may be provided for adjusting the sensitivity of the thin flexible diaphragm. Alternatively, the movable wall constituted by the pressure rate of rise responsive element may comprise a disc or plate made of magnetic material normally held in the inoperative position by a permanent magnet.

The second movable wall may comprise a flexible diaphragm extending across the housing and spaced from the pressure rate of rise responsive element, or it may comprise a bellows device.

Means may be provided to reduce the pressure of liquid surges and the like entering the housing, in order to prevent untimely operation of the device.

Two pressure rate of rise responsive elements respectively carrying co-operating electrical contacts may be 'United States Patent mounted in the housing on opposite sides of, and spaced apart from, a spacing member extending across the housing, so that shallow chambers are provided between the inner sides of the two pressure rate of rise responsive elements and opposite sides of the spacing member, respectively, a second chamber being formed on the outer sides of each pressure rate of rise responsive member by flexible walls extending across the housing.

The pressure rate of rise responsive element is fitted with a pressure leak which may be so designed that the element responds only to rates of pressure rise exceeding 5 lbs. per square inch per second, and it may have a sensitivity which is less than /2 lb. per square inch before the pressure leak is provided.

The device may be fitted with means for attaching it outside a container in which fire or explosion is likely to occur.

One form of detecting device made according to the invention comprises a cup-shaped housing across the mouth of which a thin diaphragm is mounted, a spacing member disposed in front of said diaphragm and a flexible and extendable membrane or bellows mounted across said spacing member so that said membrane or bellows transmits pressure rise to the diaphragm, which, when actuated, may close a pair of electrical contacts to actuate the explosion suppression means.

When this device is submerged in liquid, it is arranged to be hermetically sealed to exclude liquid from the pressure rate of rise responsive diaphragm and its pressure equalization orifice or hole. As explosions occurring in the vapour space of a liquid container are transmitted throughout the liquid the detecting device operates as well within the vapour space as in the liquid.

If the device is fitted in an aircraft which flies to a high altitude, when the atmospheric pressure falls the air trapped in the device expands, and if a substantial pressure is produced upon the flexible diaphragm an explosion may not operate the detector device in the initial stages.

To overcome this the displacement of the diaphragms is made small and the chamber behind the diaphragm is made shallow, as is also the space between the pressure rate of rise detecting diaphragm and the flexible diaphragm, so that the total air volume is sufiiciently small to enable the flexible diaphragm or device to expand without appreciable stress so that the pressure within the detecting device is substantially equal to the external atmospheric pressure at all practical altitudes.

In the accompanying drawings which illustrate how the invention may be carried into effect:

Figure 1 is a view in vertical section of one embodiment of the invention suitable for use at low altitudes, e. g. not in an aircraft.

Figures 2 and 3 are respectively views in vertical section and plan of another embodiment of the invention having a high degree of sensitivity and suitable for use at high altitudes e. g. in aircraft fuel tanks.

Figures 4 and 5 are respectively views in vertical section and plan of a further embodiment of the invention, suitable for use on aircraft flying at high altitudes.

Figures 6 and 7 are respectively views in vertical section and plan of another embodiment of the invention alsg suitable for use in aircraft flying at high altitudes, an

Figure 8 is a sectional view illustrating an embodiment of the invention suitable for fixing to the outside of a tank or container.

Referring first to Figure l of the drawings numeral 2 denotes a cup-shaped housing and 3 and 9 denote insulating spacer rings secured to the mouth of the housing 2.

A thin diaphragm 4 of the barometric type is secured across the mouth of the housing 2 to form a substantial chamber 5 at the rear thereof so that the diaphragm will not be greatly aflected by compression of air in the chamber 5 when external pressure variations are applied to the diaphragm. Because the diaphragm device 4 is thin or of the barometric type it is responsive to pressure of very small magnitude.

To exclude fuel and air or vapour from diaphragm 4 a flexible diaphragm 6 is clamped between the rings 3 and 9, this diaphragm 6 being protected from liquid splashing or unintentional contact by a metal wire gauze diaphragm 7 Diaphragm 4 may rest upon an insulating ring 8 to which it is clamped by the ring 9. A wire gauze 7 is clamped between ring 3 and a further ring 10, and a ring clamp 11 clamps all the rings 3, 8, 9 and 10 together and secures them to the mouth of the housing 2.

Diaphragm 4 is provided with an electric contact plate 12 secured thereto by insulating adhesive or in other manner, a pair of contact points 13 being secured in and insulated from housing 2 so that electrical connection to the contacts 13 can be made by a known type of plug and socket device.

When the above described device is placed in a tank containing liquid, e. g. petrol, it will be understood that in the case of incipient fire or explosion, the device being in the vapour space in the tank, the contacts 13 will be closed by plate 12. The rise of pressure due to the incipient fire or explosion will be transmitted through gauze 7 to flexible diaphragm 6, which, by compressing the air in the space between it and the diaphragm 4, will cause flexing of the latter to bridge the contracts 13. Normal variations in pressure are equalised, however, by a bleed hole 4a provided in diaphragm 4.

At high altitude, it will be appreciated that when the atmospheric pressure is lowered, the air in space and in the space between diaphragms 4 and 6 will expand excessively, and the device shown in Fig. 1 is therefore not suitable for use at high altitude.

Referring now to Figures 2 and 3 of the drawings, numerals 20, 20 denote a pair of identical barometric apparatus type of flexible diaphragms mounted upon an insulating spacing member 21 having grooved recesses 21a formed therein to locate the rims of the diaphragms 20. The latter are held by clamping rings 22, 22a upon which rest flexible diaphragms 23, 23a made of a suitable resilient material, and which are deeply corrugated for a purpose to be hereafter explained.

Member 21 and the rims of diaphragms 23, 23a are clamped by rings 24, 24a, themselves clamped together by clamping rings 25, 25a secured together by bolts 26. To facilitate mounting of the device lugs 27 are formed integrally or otherwise with rings 25.

Diaphragms 20 may have a thickness between .004 and .010 and a pair of contacts 20a are secured centrally thereto so that the spacing between the points of contact is about .014". To permit breathing of air through diaphragms 20 due to expansion and contraction of air trapped between diaphragms 20, 23 and 23a small bleed tubes 20]) are provided in the diaphragms as shown.

To assist in damping out vibration of diaphragms 20, spacing member 21 is provided with a central flange 21b disposed between diaphragms 20, the flange being apertured centrally as shown to permit contacts 20a to close.

If the device according to Figure 2 is submerged in liquid the hydraulic pressure on diaphragms 23, 23a will be counteracted by the pressure of the air trapped between the diaphragms without risk of contacts 20a closing, bleed tubes 20b permitting pressure equalization.

Liquid surges or sloshing may however cause inadvertent closure of the contacts, and to avoid this fine wire gauze discs 28, 28a are secured between clamping rings 25, 25a and rings 24, 24a, the mesh of the gauze being of such fineness that the force of the liquid passing through is reduced thus eliminating risk of contact closure, rapid increases in pressure due to an explosion being however transmitted effectively.

As will be seen therefore the device shown in Figure 2 constitutes a double diaphragm device having funnel or mouth piece devices in front of each diaphragm 23, 23a to concentrate and direct a pressure valve on to each diaphragm, whereby greater sensitivity of response is obtained.

Electrical connections 30, 31 are taken from diaphragms 20, 20, through small tubes 32, 33 secured in the member 21, the leads being embedded in insulation compound for sealing the air space between the diaphragms.

By this construction the diaphragm surface area exposed to a rise of pressure wave due to an explosion is double that of a single diaphragm and the distance through which contacts 20a must move before contact is made is halved, thus greatly increasing sensitivity and the speed of detection.

One disadvantage of diaphragms of the type shown is their liability to vibration, but the efiect of vibration can be reduced by inserting blown rubber pads 34 between diaphragms 20 and securing them in flange 21b.

The small space between diaphragms 20, the presence of flange 21b and the close proximity of diaphragms 23, 23a to diaphragms 20 results in the confinement of a small volume of air between the diaphragms 23, 23a and the deep corrugations on the latter permit expansion of these diaphragms at high altitudes (low atmospheric pressure) until they fill the space behind gauge shields 28 and 28a.

By this means the necessity for pressure equalisation of the type disclosed in British patent specification No. 643,188 is avoided, and the detector device may be disposed in the vapour space in fuel tanks, or submerged below the liquid level, and it may be used at ground level at normal atmospheric pressure, or at high altitude at greatly reduced pressures.

The embodiment of the invention shown in Figures 4 and 5 incorporates a single diaphragm 40 of relatively small diameter which operates with a snap action. In these figures the parts 23, 24, 26, 27, 28 are similar to correspondingly numbered parts in Figures 2 and 3.

The external rim of diaphragm 48 is clamped by an insulation ring 41, engaging the rim of the diaphragm by insulating rings 42.

The various parts are assembled upon a base casting or fitting 43 provided with lugs for engagement by the bolts 26. The fitting 43 has an insulating bush 44 se cured therein by studs 45. A flanged screw threaded thimble device 46 is screwed into bush 44 and is adapted to engage a contact stud 40a secured centrally in diaphragm 40. By this arrangement tension can be applied to diaphragm 40 so that this becomes cocked or poised near its snap-over position, whereby the sensitivity of the diaphragm can be regulated and the diaphragm can be preset to operate at pressures of very small magnitude.

Device 46 has an insulated flanged stud 47 secured therein by screws 48, this stud carrying a central contact screw 49 which can be adjusted to limit the travel of diaphragm 40. Screw 49 is provided with securing nuts ,Which lock the screw after it has been set and forms a terminal for a lead from a twin cable 51, the other lead being connected to insulated terminal screw 52 connected to diaphragm 40 by a fiexible lead 53.

Fitting 43 is provided with a screw threaded boss 43a whereby the device as a whole can be secured to the walls of an aircraft fuel tank denoted 54 by means of lock nuts 55, a resilient washer 56 being provided to form a liquid tight seal.

A cap device 57 secured by studs 45 forms a seal for the terminal devices upon fitting 43, a sealing washer 58 being provided as shown.

The space behind diaphragm 40 is small and the volume of alr trapped is not large, but the space is sufficient to permit the diaphragm 40 to snap over to make electrical contact when a rapid rise of pressure is applied to diaphragm 23.

The air volume between diaphragms 40 and 23 is also small under normal conditions, but when the external pressure is lowered diaphragm 23 expands to provide a comparatively large space for expansion of trapped air, so that the appliance is suitable for use at high altitudes.

Figures 6 and 7 show at detecting device comprising a magnetic contact disc or cap 60 held upon an annular lip 61 formed in a ring 62 by a permanent bar magnet 63 secured by a bracket device 64 to a mounting plate 65.

The flexible portion of the device is a metal bellows device 6601? known type secured at one end to plate 65 by solder1ng or the like, the open end of the bellows being fitted with a cup shaped device 67 which substantially fills the interior of the bellows and in normal conditions rests in close proximity to plate 65.

The device is shown secured for example to the wall of an aircraft fuel tank of a rubber-like material type denoted 68, to which plates 62 and 65 are clamped by bolts 69 (see Fig. 7) to external clamping plate 70.

The magnetic disc 60 is contacted by contact point 71 carried by contact spring 71a clamped by insulation rings 72 and 73.

A double point contact spring 74a having contact points 74 spaced about .015" from discs 60 is similarly clamped between insulation rings 72, 73.

Contact spring 71a is provided with an insulated contact terminal stud 75 and spring 74a with stud 76 by which means an electrical circuit can be closed when disc 60 is blown off its seating by an explosion, bellows 66 with cup 67 transmitting the rise of pressure impulse to the disc while air behind disc 60, confined by plate 70, is slightly compressed. Bleed hole 60a permits slow pressure variations which do not therefore atfect the disc.

As will be appreciated the air confined by cup 67, bellows 66 and plate 70 expands at high altitude, bellows 66 therefore extending to permit pressure equalization between the external atmosphere and the air in the detector appliance.

In certain types of plant such as electro-detarrers for use in gas works, exposure of the detecting device to the gases and acids is not desirable and in this case a detector device, according to the invention, may be mounted externally to the container or tank, which is usually a vessel of about four to eight feet in diameter, and about 20 to 50 feet in height.

In this case a hole is formed in the side of the vessel to which an appliance as shown in Figure 8 may be secured. The appliance comprises a funnel shaped fitting denoted a to which is secured a cup shaped fitting b secured thereto by means of bolts or the like, a flexible diaphragm a being secured between the flanges of parts a and b by a ring e. The fitting b is bored through in the bottom, the wall of the bore being tapped and adapted to be engaged by a screwed boss 43a, a liquid tight joint being formed by washer 56 between parts 43 and the bottom of fitting b. The fitting b is further provided with a flanged boss 1, forming an enclosure for boss 43a, cover 57 and other parts, the boss being fitted with a closure cover g secured thereto by bolts h, a sealing washer i being clamped between these parts to form a hermetical seal.

A known type of conductor cable j with known types of fittings k are secured in boss 1 to connect terminals 52 and 49 to associated electrical connections.

Diaphragm d may be of any material resistant to acid, such as lead metal foil, plastic or fabric materials having corrosion resisting properties.

It is assumed that the pressure in the vessel is substantially atmospheric but if not the pressure in fitting b may be increased to the same value as that in the vessel, minor variations due to passage of gases through the vessel being equalized by diaphragm d. If, on the other hand, the vessel is normally operated at a pressure higher than atmospheric, then a venting orifice or a selfclosing slit in diaphragm d will permit equalization of the pressure in the detecting detector containing chamber with that in the vessel.

If an explosion occurs in the vessel the pressure rise is transmitted through diaphragms d and 23 to diaphragm 400 which, by means of contact stud 400a, makes contact with stud 49, thereby operating the suppressing device.

In order to detect explosions in the initial stages it is necessary that the diaphragm or disc devices should be adjusted to respond at less than one third of a pound per square inch static pressure when the devices are sealed before the bleed holes or orifices are provided.

This degree of sensitivity may be obtained in the double diaphragm device (Figs. 2 and 3) by making the diaphragms of a thickness of the order of .004" and of a suitable material such as beryl ium copper.

In the case of the snap action diaphragm (Figs. 4 and 5) thickness is not so important and .010" is suitable, as this can be adjusted by tensioning as above described.

In the diaphragm type of device (Figs. 2 and 3) the spring action required is obtained during the forming process by varying the degree of hardness of the material.

The degree of response to rise of pressure is critical and a rate of five pounds per square inch per second is found desirable.

This is obtained by providing suitable size bleed tubes or holes in the diaphragms; tubes having a length of A3" and a bore of .010" give this degree of sensitivity with the double diaphragm detector (Figs. 2 and 3) and a hole of the order of .015 may be suitable in cases where single diaphragms are provided.

What I claim and desire to secure by Letters Patent is:

l. Apparatus for detecting rise in pressure due to an incipient fire or explosion, said apparatus comprising a housing, a pressure rate of rise responsive element extending across said housing providing a fluid chamber on one side of said element, a movable wall extending across said housing spaced from said element on the other side thereof and unconnected therewith and providing a second fluid chamber in said housing between said element and said wall, whereby pressure variations applied to said movable Wall on the side thereof remote from said pressure rate of rise responsive element will be communicated by said movable wall to the fluid medium within the chambers in said housing, actuation of said pressure rate of rise responsive element occurring only when said pressure variations involve abnormal rates of rise.

2. Apparatus for detecting rise in pressure due to an incipient fire or explosion, said apparatus comprising a housing having at least a portion of an exterior wall movable in response to variations in external pressure, a pressure responsive element disposed with-in said housing in spaced-relation to said movable wall portion and dividing the interior of said housing into two fluid containing chambers, restrictive orifice means establishing communication between said two chambers to permit substantially immediate equalization of fluid pressure therebetween and thereby to prevent movement of said pressure responsive element with movement of said movable wall portion when the rate of external pressure rise causing said movement does not exceed a predetermined 3. Apparatus according to claim 2 in which said pressure responsive element comprises a thin flexible diaphragm.

4. Apparatus according to claim 2 in which said pressure responsive element comprises a thin flexible diaphragm, and further comprising means for adjusting the sensitivity of said diaphragm to the resultant pressure applied thereto.

5. Apparatus according to claim 2 in which said pressure responsive element comprises a plate of magnetic material, and further comprising a magnet arranged normally to hold said plate in inoperative position.

6. Apparatus according to claim 1 in which said movable wall comprises a flexible diaphragm extending across said housing.

7. Apparatus according to claim 1 in which said movable wall comprises a bellows device.

8. Apparatus according to claim 1 further comprising means for preventing untimely operation of the device by the pressure of liquid surges and the like.

9. Apparatus according to claim 1 further comprising a gauze disc fitted across the inlet to the housing to reduce the pressure of liquid surges entering the housing, in order to prevent untimely operation of the device.

10. Apparatus according to claim 1 in which said housing is of cup-shape to concentrate and direct pressure variations upon said movable wall.

11. Apparatus for detecting rise in pressure due to an incipient fire or explosion for use in combination with electrically operated explosion or fire suppressing means comprising, in combination, a housing, a spacing member extending across said housing between the ends thereof, a first movable pressure rate of rise responsive element fitted in said housing on one side of said spacing mem her, a second movable pressure rate of rise responsive element fitted in said housing on the other side of said spacing member, said pressure elements being spaced somewhat from said spacing member so as to form shallow chambers between the inner sides of said pressure elements and opposite sides of said spacing member, complementary electrical contacts carried respectively on said pressure elements, said spacing member having an aperture by means of which said contacts can make contact when the device is operated, a first flexible wall extending across said housing on the side of one of said pressure elements remote from said spacing member, a second flexible wall extending across said housing on the side of the other of said pressure elements remote from said spacing member, said first and second flexible walls being spaced respectively from the adjacent pressure element and being adapted respectively to transmit pressure variations in the surrounding medium to said pressure rate of rise responsive elements to actuate these latter to close said contacts when said pressure variations involve abnormal rates of rise.

12. Apparatus according to claim 11 in which said housing comprises two ring members and further comprising means for connecting said ring members together and for securing thereto said pressure rate of rise responsive elements, said spacing member, and said first and second flexible walls.

13. Apparatus according to claim 2 in which said restrictive orifice is so designed that the pressure responsive element responds only to rates of external pressure rise exceeding lbs. per square inch per second.

14. Apparatus according to claim 11 in which said pressure rate of rise responsive elements are fitted respectively with pressure leaks so designed that these elements respond only to rates of pressure rise exceeding 5 lbs. per square inch per second.

15. Apparatus according to claim 2 in which said restrictive orifice is so designed that the pressure responsive element responds only to rates of external pressure rise exceeding 5 lbs. per square inch per second, the sensitivity of the pressure responsive element to resultant pressure applied thereto being less than /2 lb. per square inch.

16. Apparatus according to claim 11 in which said pressure rate of rise responsive elements are fitted respectively with pressure leaks so designed that these ele ments respond only to rates of pressure rise exceeding 5 lbs. per square inch per second the sensitivity of the pressure rise responsive elements before the pressure leaks are provided being less than /z lb. per square inch.

17. Apparatus according to claim 1 further comprising means for attaching it outside a container in which incipient fire or explosion is to be detected.

18. A pressure rate of rise detecting device comprising a housing having at least a portion of an exterior wall movable in response to variations in external pressure, a pressure responsive element disposed within said housing in spaced-relation to said movable wall portion and dividing the interior of said housing into two fluid containing chambers, restrictive orifice means establishing communication between said two chambers to permit substantially immediate equalization of fluid pressure therebetween and thereby to prevent movement of said pressure responsive element with movement of said movable wall portion when the rate of external pressure rise causing said movement does not exceed a predetermined limit, and electrical contacts disposed within the chamber remote from said movable wall portion and adapted to be closed upon movement of said pressure responsive element in response to a rate of external pressure rise above said predetermined limit.

19. A pressure rate of rise detecting device according to claim 18, wherein said electrical contacts comprise a pair of contacts adjacent said pressure responsive element, and further comprising an electric circuit including said pair of contacts for actuating fire or explosion suppressing means, and a conductive member carried by said pressure responsive element for completing the circuit between said contacts.

20. A pressure rate of rise detecting device according to claim 18 wherein said pressure responsive element comprises a diaphragm of the snap action type.

21. Apparatus for detecting rise in pressure according to claim 1, wherein said chambers are shallow providing a small total fluid volume whereby said pressure rate of rise responsive element is enabled to deform without ap- 5 preciable stress.

References Cited in the file of this patent UNITED STATES PATENTS 

